The development of fuels from renewable agricultural sources is currently and will likely continue to be important in meeting future energy demands and reducing the production of greenhouse gas emissions from fossil carbon sources. Current “biofuels” under development include “biodiesel,” which is derived via fatty acid synthesis from vegetable oil and ethanol fermented from sucrose obtained from plants such as corn and sugarcane.
More diverse and advanced biofuels and bio-products may be developed by exploiting metabolic pathways other than fatty acid synthesis and fermentation. For example, plants and bacteria use the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway to synthesize isoprenoids such as isoprene (C5H8, 2-methyl 1,3-butadiene) and pinene (C10H16, bicyclic monoterpene) as well as other terpenoids (see FIG. 1). End products of the MEP pathway are isopentyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). Isoprene, which is a monomer of natural rubber and a precursor for synthetic rubber and thermoplastic elastomers, is made from DMAPP by the enzyme isoprene synthase (IspS). Pinene, a liquid bicyclic monoterpene, is made from IPP and DMAPP via geranyl diphosphate synthase (GPPS) and mono-terpene synthase (mono-TPS).
Isoprenoids are currently made industrially from petrochemicals and then converted into synthetic polymers, high-density liquid biofuels, and other materials (Mark et al., In: Encyclopedia of Polymer Science and Technology V7:782-854 (1967)). For example, pinene can be dimerized to exo-tetrahydrodicyclopentadiene, the energy value of which is 141,745 BTU/gallon (or 42.1 MJ kg−1), nearly identical to that of the tactical jet fuel, JP-10 (Harvey et al., Energy Fuels 24:267 (2009)). Isoprene and, more particularly, the polymer cis-polyisoprene find utility in the production of specialty items such as vitamins, pesticides, pharmaceuticals, flavors, epoxy hardeners, and a variety of products containing elastic substances.
As petrochemical sources for industrial feedstocks and fuels become scarce, demand for alternative, carbon-neutral methods of producing isoprenoid feedstock chemicals will increase. Accordingly, there is a need for improved methods of producing isoprene and pinene.